Since organic electroluminescent devices are self-luminescent devices, they are bright and excellent in visibility as compared with liquid-crystalline devices and capable of giving clear display, so that the organic electroluminescent devices have been actively studied.
In 1987, C. W. Tang et al. of Eastman Kodak Company put an organic electroluminescent device using organic materials into practical use by developing a device having a multilayered structure wherein various roles are assigned to respective materials. In particular, they formed a lamination of a fluorescent material capable of transporting electrons and an organic material capable of transporting holes, so that both charges are injected into the layer of the fluorescent material to emit light, thereby achieving a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or lower (see e.g., Patent Documents 1 and 2).
To date, many improvements have been performed for practical utilization of the organic electroluminescent devices, and high efficiency and durability have been achieved by an electroluminescent device wherein an anode, a hole-injection layer, a hole-transport layer, a light-emitting layer, an electron-transport layer, an electron-injection layer, and a cathode are sequentially provided on a substrate, to further segmentalize various roles (see e.g., Non-Patent Document 1).
Moreover, for the purpose of further improvement of luminous efficiency, utilization of triplet exciton has been attempted and utilization of a phosphorescent material has been investigated (see e.g., Non-Patent Document 2).
The light-emitting layer can be also prepared by doping a charge-transport compound, generally called a host material, with a fluorescent material or a phosphorescent material. As described in the above-mentioned Non-Patent Documents 1 and 2, the choice of the organic materials in organic electroluminescent devices remarkably affects various properties such as efficiency and durability of the devices.
In the organic electroluminescent devices, the charges injected from the both electrode are recombined in the light-emitting layer to attain light emission. However, since the mobility of holes is higher than the mobility of electrons, a problem of reduction in efficiency caused by a part of the holes passing through the light-emitting layer arises. Therefore, it is required to develop an electron-transport material in which the mobility of electrons is high.
A representative light-emitting material, tris(8-hydroxyquinoline)aluminum (hereinafter referred to as Alqa) is commonly used also as an electron-transport material. However, since the mobility of electrons in the material is low and the material has a work function of 5.6 eV, it cannot be considered that the material has enough hole-blocking capability.
As a technique to prevent the passing of a part of holes through the light-emitting layer and to improve probability of charge recombination in the light-emitting layer, there is a method of inserting a hole-blocking layer. As hole-blocking materials, there have been hitherto proposed triazole derivatives (see e.g., Patent Document 3), bathocuproine (hereinafter referred to as BCP), a mixed ligand complex of aluminum (e.g., bis(2-methyl-8-qunolinolate)-4-phenylphenolato aluminum (III) (hereinafter, referred to as BAlq)) (see e.g., Non-Patent Document 2), and the like.
On the other hand, as an electron-transport material excellent in hole-blocking ability, there is proposed 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (hereinafter referred to as TAZ) (see e.g., Patent Document 3).
Since TAZ has a work function as large as 6.6 eV and thus exhibits a high hole-blocking ability, it is used as an electron-transport hole-blocking layer to be laminated onto the cathode side of a fluorescence-emitting layer or phosphorescence-emitting layer prepared by vacuum deposition, coating or the like, and contributes to increase the efficiency of organic electroluminescent devices (see e.g., Non-Patent Document 3).
However, TAZ has a great problem of having low electron-transport property, and it is necessary to prepare an organic electroluminescent device in combination with an electron-transport material having a higher electron-transport performance (see e.g., Non-Patent Document 4).
Further, BCP has a work function as large as 6.7 eV and a high hole-blocking ability, but has a low glass transition point (Tg) which is 83° C., so that it is poor in thin-film stability and thus it cannot be considered that it sufficiently functions as a hole-blocking layer.
All the materials are insufficient in thin-film stability or are insufficient in the function of blocking holes. In order to improve device characteristics of the organic electroluminescent devices, it is desired to develop an organic compound which is excellent in electron-injection/transport performances and hole-blocking ability and is highly stable in thin-film state.
As a compound in which these characteristics have been improved, a compound having an anthracene ring structure and benzimidazole ring structure is proposed (see, e.g., Patent Document 4).
However, although devices where such compounds are used in electron injection layer and/or electron transport layer are improved in luminous efficiency or the like, it is not sufficient, and further lowering in driving voltage and enhancing in luminous efficiency, in particular, enhancing in current efficiency are required.